The present invention concerns a method for controlling the braking system of a vehicle when leaving a standstill state where parking brake is applied to a driving state where brakes are released. This method is adapted to vehicles which are equipped with an electronically controlled parking brake system.
In contrast to a mechanical parking brake, which is actuated and disengaged by the driver using a hand-brake lever, a parking brake that is electronically controlled can be automatically released. The park brake force generated in an electronically controlled parking brake is generally high enough to prevent the vehicle from rolling away in any circumstances and whatever the physical parking, conditions under which the vehicle is stopped or parked.
US-A-2008/0149437 discloses a method for automatically braking a vehicle equipped with an electronically controlled parking brake. The braking system is pneumatic and includes a parking brake and a service brake. The parking brake is released when increasing the pressure inside a parking brake cylinder and the service brake is actuated when increasing the pressure in a service brake cylinder. For safety reasons, when leaving the standstill state, the parking brake is released only when the service brake is already producing a minimum braking effect. In this way, there is no risk that the vehicle rolls away, but this entails an extra braking force applied by the braking system. This extra braking force can be determined by summing up the park brake force already applied with the service brake force applied before releasing the parking brake. Occurrence of this extra braking force is better known as «compound effect».
The major drawback of compound effect is that it implies additional constraints within the brake, and consequently, an accelerated fatigue of the brake.
US-A-2010/0211281 discloses a method for applying a parking brake of a vehicle depending on the roadway conditions. This method teaches to adjust the park brake force in function of the conditions in which the vehicle is parked. For example, the slope of the road and the weight of the vehicle affect the park brake force needed to maintain the vehicle in a standstill state. Moreover, weather conditions are also taken into account when determining the suited park brake force. This method does not intend to reduce the compound effect occurring during the transition between the parking brake mode and the service brake mode. Therefore, unless using oversized brakes able to accept compound additional constraints, a compound effect will cause a premature wear of the brake.
It is desirable to provide a method for controlling the braking system of a vehicle in which the compound effect is reduced.
An aspect of the invention concerns a method for controlling, the braking system of a vehicle when switching from a standstill state where parking brake is applied to a driving state of the vehicle. This braking system includes a parking brake and a service brake. The addition of the park brake force and the service brake force gives a total brake force. According to the invention, this method includes at least the following steps:
a) detecting at least one starting condition of the vehicle,
b) determining at least one parking condition of the vehicle,
c) assessing whether said parking condition is favorable or unfavorable,
d) if said parking condition is considered to be favorable at step c), starting to release the parking brake, and
e) after step c), starting to apply the service brake.
Thanks to an aspect of the invention, when the parking conditions are favorable, it is possible to partially release the park brake force without the vehicle rolling away. So, the park brake can start being released before starting to apply the service brake. Since the parking brake is partially released when it is started to apply the service, brake, the accumulation of the park brake effort and the service brake effort does not exceed the park brake force initially applied in standstill state of the vehicle that is to say before step d). In other words, there is no more compound effect. By limiting the occurrence of compound effect all along the brake life, the fatigue on the brake is drastically reduced.
According to further aspects of the invention which are advantageous but not compulsory, such a method may incorporate one or several of the following features:
The following steps f) and g) are implemented:
step f) if said at least one parking condition is considered to be unfavorable at step c), starting to apply the service brake, and
step g) after step f), starting to release the parking brake. (Currently Amended) The braking system of the vehicle is pneumatic and the release of the parking brake is started by increasing the pressure inside a parking brake chamber and the application of the service brake is started by increasing the pressure in a service brake chamber. More precisely, the release of the parking brake is started by increasing from zero (relative pressure) the pressure inside a parking brake chamber and the application of the service, brake is started by increasing from zero (relative pressure) the pressure in a service brake chamber.
if said parking condition is considered to be favorable at step c), the pressure in the service brake chamber and the pressure in the parking brake chamber are controlled such that, after step d), the total brake force remains inferior or equal to the park brake force initially applied and such that the total brake force remains higher than a minimal brake force necessary to maintain stopped the vehicle. In the present application, the expression “park brake force initially applied” refers to the park brake three applied during the standstill state of the vehicle.
If said parking condition is considered to be favorable at step c), the pressure in the service brake chamber is built up to a target pressure, which is determined such that the total braking force is lower than the park brake force initially applied.
If said parking condition is considered to be favorable at step c), the difference between the total brake force and the park brake force initially applied may vary, after step d), depending on said at least one parking condition or depending on at least one other parking condition and is preferably maximal when the vehicle is stopped on a flat surface.
The maximal difference occurring after step d) between the total brake force and the park brake force initially applied, is superior to an extra braking force occurring after step f) and said extra braking force being equal to the maximum difference, occurring after step f), between the total brake force and the park brake force initially applied when the total brake force is higher than the park brake force initially applied.
The step e) occurs after a period of time has elapsed since step d), this period of time is determined so that the total brake force remains higher than the minimal brake force that is necessary to maintain stopped the vehicle and so that the total brake force remains lower than the park brake force initially applied.
The period of time is preferably comprised between 100 ms and 600 ms.
The duration of the period of time after which step e) occurs depends on the pressure in the parking brake chamber and step e) occurs when the pressure in the parking brake chamber reaches a threshold value.
When said parking condition is considered to be favorable at step c) the pressure in the parking brake chamber and the pressure in the service brake chamber are automatically controlled by at least one control unit during at least steps d) and e).
Said parking condition takes into account the slope gradient of the road or of surface where the vehicle is parked on and/or the weight or the load of the vehicle.
The vehicle is a truck, several parking conditions are determined in step b) and in the parking conditions are favorable when the truck is parked on a road with a slope inferior to 8% and when the truck is loaded with less than 50% of its total capacity.
The vehicle is a truck and said parking condition is favorable when the truck is parked on a road with a slope inferior to 3%.
The parking conditions take into account weather conditions.
The step a) detection of said at least one starting condition comprises detection of the user in the cabin of the vehicle and/or detection of engine start and/or detection of manual gear selection and/or detection of a drive position of the gearbox.
After step e) or g), the method further comprises the step h) and i) where step h) consists in or comprises detecting a driver request for starting the vehicle and, following step h), step i) it consists in or comprises releasing the service brake and, if it is not already released, releasing the parking brake.
Preferably, step h) consists in or comprises detecting a depression of the accelerator pedal by the driver.